Download Lecture 2 Nucleic Acid Structure

Lecture 2- Nucleic Acid Structure
Forces stabilizing DNA
Forces stabilizing DNA
Van der Waals Interactions
Hydrophobic Effect
-Electrostatic interaction between two mutually induced dipoles
-Describes tendency of hydrophobic molecules to transfer from
aqueous phase to organic phase
E=
-B
+
r6
B (A6.kcal/mol)
Atom
Lennard-Jones Potential
H
46
O
220
OH
470
CH2
1160
S
3760
SH
4560
A
r12
B is a function of polarizability
Benzene crystal VdW Energy
-9.9 kcal/mol
H O
H
O H
H
H
O
H
(1) Iceberg-like structure of water around organic
solute: All four hydrogen bonds formed/ high
order-entropic rerdty
(In liquid water:
roughly 1/2 hydrogen bonds formed)
(2) poor polarizability of oxygen
organic molecule tends to decreased VdW interaction between water and
aggregate in water to
organic solute
minimize hydrophobic effect
also drives hydrophobic effect
E (kcal/mol)
0
VdW radii (Å)
Atom
Van der Waals radius
1.5
RO H
O
CH4-CH4
R2
R1
3.7 Å
r
1.5
S R
1.5
O
O
-0.2
R1
N
H
R2
1.55
1.65
CH (tet)
1.85
CH (tri)
1.7
R
R1
S
R2
1.8
=P
-Energetic contribution due to substituent
-R groups have a constant additive effect on hydrophobicity of parent
compound in absence of strong resonance/inductive effects
Group
NH3+
Solubility in octanol
Solubility in water
G = transfer from n-octanol to water
G = RT log P/P0
O
R
VdW distance = VdWradii
-Measurement of hydrophobicity is partitioning of molecule between n-octanol
and H2O
Solubility in octanol
= P0
S H
Solubility in water
G (kcal/mol)
-CH4
0.68
-Et
1.36
-propyl
2.05
-isopropyl
1.77
Group
-benzyl
-OH
O
G (kcal/mol)
3.6
-1.58
-1.65
N
H
1 A2 of hydrophobic surface ca. 24 cal/mol stabilization when removed from H2O
-Hydrophobic Effect can be relatively non-specific when molecules bind to
protein or DNA
Lecture 2- Nucleic Acid Structure
Environmental Effect on pKas
Forces stabilizing DNA
pKa
% dioxane in H2O
Electrostatic Interaction
-Proportional to 1/r; Long distance interaction compared to VdW
e1 e2
E=
H
Dr
e
0
20
45
70
acetic acid (CH3CO2H)
4.76
5.29
6.31
8.34
glycine (NH3+)
9.8
9.3
8.5
7.42
3.3 Å
D is dielectric constant
-Leveling Effect
-Point charge induces dipoles in solvent which together with
counterions induce field to neutralize charge
D water ~ 80
-1.3 kcal/mol (very small)
D pentane ~ 2
--50 kcal/mol
Metal ion coordination stabilizes DNA
Mg2+
-So electrostatic interactions are far more important in organic solvent
than water.
HN
pKa2 ~ 3
Mn2+
Zn2+
Cd2+
Cu2+
(Hard)
(Soft)
Bind
-OH/-PO4-
Bind
Bases
Duplex DNA
G = -RT logK
G ca. 8.4 kcal/mol
pKa ~ 6
N
G = -RT logK = -1.4 logK kcal/mol (room temp)
-1.4 kcal/mol for Product/Substrate = 10
O
O
5'
B1
G= = -1.4 log(K2/K1) at room temp
G
G=1
G=2
G
product
Progress of Reaction
B3
O
O
O P OO
O
O P O
O
O
G=
3'
5'-GCAT......
3'-CGTA......
3'
O
transition state
substrate
Ni2+
pKa1 ~ 8.8
NH
HN
Co2+
B2
B4
Always specify
directionality of DNA
sequence
5'
O
O
O
= -1.4 kcal/mol for a 10 fold
difference in rate
A
A-T pair:
Minor Groove
D
O
NH
N
H2N
A
N
N
N
N
O A
A
Major Groove
Lecture 2- Nucleic Acid Structure
Duplex DNA (continue)
Helix formation is cooperative
Major Groove
NH2
A
O
A
N
G-C pair:
N
N
HN
N
N
O
A
H2N
A
U
U
U
U
U
K= S
A U
A
U
A
U
A
U
A
U
K=S
A U
A U
A
U
A
U
A
U
......
D
Minor Groove
B form DNA (dominant form)
-Right handed under physiological conditions
-10 bp/turn
-3.3 Å/bp
-helix axis through center of bp
-anti
-C2' endo
-wide major groove - binds peptides
-narrow minor groove - binds planar molecules
A form DNA
-Right handed (more contact)
-11 bp/turn
-2.3 Å/bp (short/broader)
-24 Å/turn
-major groove (deep narrow)/minor groove (broad shallow)
-anti
-C3' endo
Z form DNA
-Left handed
-12 bp/turn
-3.8 Å/bp
-anti C/Syn G
-no major groove/minor groove (deep narrow)
A form RNA
-C3' endo
-RNA can adapt more distinct structures than DNA
Single-stranded RNA
A form
DNA-RNA duplexes
A form
DNA duplexes
A
A
A
A
A
A form
= 10-3 M-1 at 0 oC
S = 10
at 0 oC
-Helix coil transition is highly cooporative and nucleates by 3 bps
Melting Temperature Tm
-Tm characterizes the stability of
double helix
ssDNA
A260
D
-Melting duplex to singlestranded is accompanied by an
increase in absorbance
-Tm depends on length, ionic
strength and GC/AT ratio
Tm calculator:
+ 2 oC A-T pair
+ 4 oC A-T pair
duplex
Tm
RNA secondary structure
single strand
A-form
double helix single nt bulge
hairpin loop
mismatch pair
symmetric
internal loop
-Secondary RNA structure
can be predicted
three nt bulge
asymmetric
internal loop
-tertiary RNA structure is
hard to be predicted
Lecture 2- Nucleic Acid Structure
DNA-binding small molecules
Electrostatic Interaction
-target phosphated sugar backbone
H2
N
H3N
N
H2
Higher order of DNA structure
-Supercoiling
Linear DNA
NH3
spermine
Intercalation
H2N
Unwound circle
-targets G-C rich regions
NH2
-Kd 10-6 M-1
-binds DNA from the narrow groove
N
negative supercoil DNA
(Right-handed)
Br
-2 supercoil (neg) ~ 2 linear base
Enzymes to control winding and unwinding of DNA
-Topoisomerase (wind and unwind DNA)
target for cancer drug
G
G
C
C
G
H2N
Packing of chromosomal DNA
C
NH2 3.3 Å
BrH
G
helix becomes slightly unwinded
-DNA is packaged into chromatin in mammalian cells
-duplex DNA tightly binds to 5 histones
Cancer drug - Doxorubicin
O
DNA
H2A X 2
packing ratio
H2B X 2
is 104 in
H3 X 2
human
H4 X 2
genome
10-20 kDa
N
bind
3.3 Å
C
OH
O
OH
OH
55 Å
H1
680 Å into ~110 Å leads to
packing ratio of 7
O
O
OH OSugar
Doxorubicin
110 Å
-Modulate duplex DNA and histone Interaction
-acetylation of lysine (positive charge of histone proteins~ neutralized)
-other post-translational modifications (PTMs) exist for modulating
histone/DNA interaction
-DNA is a popular target for cancer drugs as rapidly proliferating cells
need to replicate DNA faster
-binds DNA from minor groove
-cancer drug
-when Topo unwinds
DNA, dox stablizes single
strand DNA intermediate